Coordination of cell proliferation and cell death is required for normal development and tissue homeostasis in multicellular organisms. A defect in the normal coordination of these two processes is a fundamental requirement for tumorigenesis.
Progression through the cell cycle is highly regulated, requiring the transit of numerous checkpoints (for review, see Hunter, 1993). The extent of cell death is physiologically controlled by activation of a programmed suicide pathway that results in morphologically recognizable form of death termed apoptosis (Jacobson et al, 1997; Vaux et al., 1994). Both extra-cellular signals, such as tumor necrosis factor, and intracellular signals, like p53, can induce apoptotic cell death. Although many proteins involved in apoptosis or the cell cycle have been identified, the mechanisms by which these two processes are coordinated are not well understood.
It is well established that molecules which modulate apoptosis have the potential to treat a wide range of conditions relating to cell death and cell proliferation. For example, such molecules may be used for inducing cell death for the treatment of cancers, inhibiting cell death for the treatment of neurodegenerative disorders, and inhibiting or inducing cell death for regulating angiogenesis. However, because many biological pathways controlling cell cycle and apoptosis have not yet been fully elucidated, there is a need for the identification of biological targets for the development of therapeutic molecules for the treatment of these disorders.
PML Nuclear Bodies
PML nuclear bodies (PML-NBs), also known as PODs (PML oncogenic domains), ND10 (nuclear domain 10) and Kr bodies, are discrete subnuclear domains that are specifically disrupted in cells from acute promyelocytic leukemia (APL), a distinct subtype of human myeloid leukemia (Maul et al., 2000; Ruggero et al., 2000; Zhong et al., 2000a). Their name derives from their most intensively studied protein component, the promyelocytic leukemia protein (PML), a RING finger IFN-inducible protein encoded by a gene originally cloned as the t(15; 17) chromosomal translocation partner of the retinoic acid receptor (RAR) locus in APL. In APL cells, the presence of the leukemogenic fusion protein, PML-RAR, leads to the disruption of PML-NBs and the delocalization of PML and other PML-NB proteins into aberrant nuclear structures (Zhong et al., 2000a). Treatment of both APL cell lines and patients with retinoic acid, which induces the degradation of the PML-RAR oncoprotein, results in relocalization of PML and other NBs components into PML-NBs and complete remission of clinical disease, respectively. The deregulation of the PML-NBs by PML-RAR thus appears to play a critical role in tumorigenesis. The analysis of mice, where the PML gene was disrupted by homologous recombination, has revealed that PML functions as a tumor suppressor in vivo (Wang et al., 1998a), that is essential for multiple apoptotic pathways (Wang et al., 1998b). Pml −/− mice and cells are protected from Fas, TNFα, ceramide and IFN-induced apoptosis as well as from DNA damage-induced apoptosis. However, the molecular mechanisms through which PML modulates the response to pro-apoptotic stimuli are not well understood (Wang et al., 1998b; Quignon et al., 1998). Recent studies indicate that PML can participate in both p53-dependent and p53-independent apoptosis pathways (Guo et al., 2000; Fogal et al., 2000). p53-dependent DNA-damage induced apoptosis, transcriptional activation by p53 and induction of p53 target genes are all impaired in PML −/− primary cells (Guo et al., 2000). PML physically interacts with p53 and acts as a transcriptional co-activator for p53. This co-activatory role of PML is absolutely dependent on its ability to recruit p53 in the PML-NBs (Guo et al., 2000; Fogal et al., 2000). The existence of a cross-talk between PML- and p53-dependent growth suppression pathways implies an important role for PML-NBs and PML-NBs-associated proteins as modulators of p53 functions. In addition to p53, the pro-apoptotic factor Daxx could be another important mediator of PML pro-apoptotic activities (Ishov et al., 1999; Zhong et al., 2000b; L1 et al., 2000). Daxx was initially identified by its ability to enhance Fas-induced cell death. Daxx interacts with PML and localizes preferentially in the nucleus where it accumulates in the PML-NBs (Ishov et al., 1999; Zhong et al., 2000b; L1 et al., 2000). Inactivation of PML results in delocalization of Daxx from PML-NBs and complete abrogation of Daxx pro-apoptotic activity (Zhong et al., 2000b). Daxx has recently been found to possess strong transcriptional repressor activity (L1 et al., 2000). By recruiting Daxx to the PML-NBs, PML may inhibit Daxx-mediated transcriptional repression, thus allowing the expression of certain pro-apoptotic genes.
PML-NBs contain several other proteins in addition to Daxx and p53. These include the autoantigens Sp100 (Sternsdorf et al., 1999) and Sp100-related protein Sp140 (Bloch et al., 1999), the retinoblastoma tumor suppressor pRB (Alcalay et al., 1998), the transcriptional co-activator CBP (LaMorte et al., 1998), the Bloom syndrome DNA helicase BLM (Zhong et al., 1999) and the small ubiquitin-like modifier SUMO-1 (also known as sentrin-1 or PIC1; for recent reviews see Yeh et al., 2000; Melchior, 2000; Jentsch and Pyrowolakis, 2000). Covalent modification of PML by SUMO-1 (sumoylation) appears to play a critical role in PML accumulation into NBs (Muller et al., 1998) and the recruitment of other NBs components to PML-NBs (Ishov et al., 1999; Zhong et al., 2000c).
Prostate Apoptosis Response-4
Prostate apoptosis response-4 (PAR4) is a 38 kDa protein initially identified as the product of a gene specifically upregulated in prostate tumor cells undergoing apoptosis (for reviews see Rangnekar, 1998; Mattson et al., 1999). Consistent with an important role of PAR4 in apoptosis, induction of PAR4 in cultured cells is found exclusively during apoptosis and ectopic expression of PAR4 in NIH-3T3 cells (Diaz-Meco et al., 1996), neurons (Guo et al., 1998), prostate cancer and melanoma cells (Sells et al., 1997) has been shown to sensitize these cells to apoptotic stimuli. In addition, down regulation of PAR4 is critical for ras-induced survival and tumor progression (Barradas et al., 1999) and suppression of PAR4 production by antisense technology prevents apoptosis in several systems (Sells et al., 1997; Guo et al., 1998), including different models of neurodegenerative disorders (Mattson et al., 1999), further emphasizing the critical role of PAR4 in apoptosis. At the carboxy terminus, PAR4 contains both a leucine zipper domain (Par4LZ, amino acids 290-332), and a partially overlapping death domain (Par4DD, amino acids 258-332). Deletion of this carboxy-terminal part abrogates the pro-apoptotic function of PAR4 (Diaz-Meco et al., 1996; Sells et al., 1997; Guo et al., 1998). On the other hand, overexpression of PAR4 leucine zipper/death domain acts in a dominant negative manner to prevent apoptosis induced by full-length PAR4 (Sells et al., 1997; Guo et al., 1998). The PAR4 leucine zipper/death domain mediates PAR4 interaction with other proteins by recognizing two different kinds of motifs: zinc fingers of the Wilms tumor suppressor protein WT1 (Johnstone et al., 1996) and the atypical isoforms of protein kinase C (Diaz-Meco et al., 1996), and an arginine-rich domain from the death-associated-protein (DAP)-like kinase Dlk (Page et al., 1999). Among these interactions, the binding of PAR4 to aPKCs and the resulting inhibition of their enzymatic activity is of particular functional relevance because the aPKCs are known to play a key role in cell survival and their overexpression has been shown to abrogate the ability of PAR4 to induce apoptosis (Diaz-Meco et al., 1996; Berra et al., 1997).
SLC/CCL21
Chemokine SLC/CCL21 (also known as SLC, CKβ-9, 6Ckine, and exodus-2) is a member of the CC (beta)-chemokine subfamily. SLC/CCL21 contains the four conserved cysteines characteristic of beta chemokines plus two additional cysteines in its unusually long carboxyl-terminal domain. Human SLC/CCL21 cDNA encodes a 134 amino acid residue, highly basic, precursor protein with a 23 amino acid residue signal peptide that is cleaved to form the predicted 111 amino acid residues mature protein. Mouse SLC/CCL21 cDNA encodes a 133 amino acid residue protein with 23 residue signal peptide that is cleaved to generate the 110 residue mature protein. Human and mouse SLC/CCL21 is highly conserved, exhibiting 86% amino acid sequence identity. The gene for human SLC/CCL21 has been localized at human chromosome 9p13 rather than chromosome 17, where the genes of many human CC chemokines are clustered. The SLC/CCL21 gene location is within a region of about 100 kb as the gene for MIP-3 beta/ELC/CCL19, another recently identified CC chemokine. SLC/CCL21 was previously known to be highly expressed in lymphoid tissues at the mRNA level, and to be a chemoattractant for T and B lymphocytes (Nagira, et al. (1997) J. Biol. Chem. 272:19518-19524; Hromas, et al. (1997) J. Immunol. 159:2554-2558; Hedrick, et al. (1997) J. Immunol. 159:1589-1593; Gunn, et al. (1998) Proc. Natl. Acad. Sci. 95:258-263). SLC/CCL21 also induces both adhesion of lymphocytes to intercellular adhesion molecule-1 and arrest of rolling cells (Campbell, et al. (1998) Science 279:381-384). All of the above properties are consistent with a role for SLC/CCL21 in regulating trafficking of lymphocytes through lymphoid tissues. Unlike most CC chemokines, SLC/CCL21 is not chemotactic for monocytes. However, it has been reported to inhibit hemopoietic progenitor colony formation in a dose-dependent manner (Hromas et al. (1997) J. Immunol. 159: 2554-58).
Chemokine SLC/CCL21 is a ligand for chemokine receptor CCR7 (Rossi et al. (1997) J. Immunol. 158:1033; Yoshida et al. (1997) J. Biol. Chem. 272:13803; Yoshida et al. (1998) J. Biol. Chem. 273:7118; Campbell et al. (1998) J Cell Biol 141:1053). CCR7 is expressed on T cells and dendritic cells (DC), consistent with the chemotactic action of SLC/CCL21 for both lymphocytes and mature DC. Both memory (CD45RO+) and naïve (CD45RA+) CD4+ and CD8+ T cells express the CCR7 receptor (Sallusto et al. (1999) Nature 401:708). Within the memory T cell population, CCR7 expression discriminates between T cells with effector function that can migrate to inflamed tissues (CCR7−) vs. T cells that require a secondary stimulus prior to displaying effector functions (CCR7+) (Sallusto et al. (1999) Nature 401:708). Unlike mature DC, immature DC do not express CCR7 nor do they respond to the chemotactic action of CCL21 (Sallusto et al. (1998) Eur. J. Immunol. 28:2760; Dieu et al. (1998) J. Exp. Med. 188:373).
A key function of CCR7 and its two ligands SLC/CCL21 and MIP3b/CCL19 is facilitating recruitment and retention of cells to secondary lymphoid organs in order to promote efficient antigen exposure to T cells. CCR7-deficient mice demonstrate poorly developed secondary organs and exhibit an irregular distribution of lymphocytes within lymph nodes, Peyer's patches, and splenic periarteriolar lymphoid sheaths (Forster et al. (1999) Cell 99:23). These animals have severely impaired primary T cell responses largely due to the inability of interdigitating DC to migrate to the lymph nodes (Forster et al. (1999) Cell 99:23). The overall findings to date support the notion that CCR7 and its two ligands, CCL19 and CCL21, are key regulators of T cell responses via their control of T cell/DC interactions. CCR7 is an important regulatory molecule with an instructive role in determining the migration of cells to secondary lymphoid organs (Forster et al. (1999) Cell 99:23; Nakano et al. (1998) Blood 91:2886).